1. FIELD OF THE INVENTION
The present invention relates to a resistor device which can be used in an electric circuit.
2. DISCUSSION OF BACKGROUND
FIG. 4 is a cross sectional view showing the structure of a conventional resistor device, which has been disclosed in e.g. the article of J. APPL. Phys. Vol. 48, No. 12 p. 5161. In FIG. 4, reference numeral 1 represents particles of ruthenium oxide (RuO.sub.2). Reference numeral 2 represents lead borosilicate glass. Reference numeral 3 represents particles of alloy comprising silver (Ag) and palladium (Pd). Reference numeral 4 represents lead borosilicate glass. Reference numeral 5 represents an alumina ceramic substrate. Reference numeral 6 represents a thick-film resistor which comprises the RuO.sub.2 particles 1 and the lead borosilicate glass 2. Reference numeral 7 represents thick-film conductors which comprise the particles 3 of alloy comprising silver (Ag) and palladium (Pd), and the lead borosilicate glass 4.
The conventional resistor device is obtained by depositing by screen printing paste including the alloy particles 3 and the glass 4, and paste including the RuO.sub.2 particles 1 and the glass 2 on the alumina ceramic substrate 5 and firing them. The thick-film conductors 7 and the thick-film resistor 6 differ from each other in the dispersion state of conductive particles. The alloy particles 3 as conductive particles are in touch with one another to form a conductive network in the thick-film conductors 7, whereas the RuO.sub.2 particles 1 as conductive particles disperses without being in touch with one another in the thick-film resistor 6. In the firing step of the thick-film resistor, the RuO.sub.2 particles 1 diffuses into the glass 2 in small amounts. As a result, the glass 2 which is inherently an insulator gains conduction to become a semiconductor having high resistance. The resistance of the thick-film resistor 6 is expressed as the sum of the resistance of the RuO.sub.2 particles and that of the glass. The RuO.sub.2 particles act like metal in terms of electronic energy though the RuO.sub.2 particles are metal oxide. It means that the RuO.sub.2 particles have a wide range of electronic energy band, and that the particles are oxide having high level of electron density. The glass 2 is melted to cause the RuO.sub.2 particles to flow in the firing step, thereby giving to the electrode portions of the conventional resistor portion a microstructure wherein the RuO.sub.2 particles and the Pd/Ag alloy particles are directly in touch with one another. In this way, electrodes are formed on the thick-film resistor 6 in the form of contact of metal to metal.
Because the thick-film conductors are deposited by screen printing, there are limitations imposed on the dimensions of the films prepared in this manner. It is in practice difficult to deposit wiring having a width of 150 .mu.m or less. For these reasons, attempts have been made to use a thin film of metal as wiring, thereby obtaining minute wiring. FIG. 5 shows a conventional resistor device in section wherein thin film conductors of copper are formed on a thick-film resistor. In FIG. 5, reference numeral 1 represents RuO.sub.2 particles. Reference numeral 2 represents lead borosilicate glass. Reference numeral 5 represents an alumina ceramic substrate. Reference numeral 6 represents a thick-film resistor which comprises the RuO.sub.2 particles 1 and the lead borosilicate glass 2. Reference numeral 8 represents the thin film conductors of copper, which are deposited by chemical copper plating in the case of FIG. 5. Reference numeral 9 represents an activation layer which is predominantly lead borosilicate glass, palladium oxide being added to it in small amounts. The surface of the thick-film resistor is coated by glass having a thickness of 0.1 .mu.m or less. This surface glass is a semiconductor having a high level of resistance as stated earlier.
Since the thick-film resistors of the conventional resistor devices are constructed as described above, the electrodes of the resistor devices have the microstructure wherein the semiconductor glass and the copper are directly in touch with each other. It means that the presence of contact of semiconductor to metal produces a barrier in the energy state of the electrons at the contacting portions. A change in connection resistance has a significant effect on the resistance of the thick-film resistor, and the resistance of the thick-film resistor will change with time.